Patent GB-943105-A (GB)
Title: Apparatus for photographic printing of colour records
Inventor: NEALE DENIS MANKTELOW
IPC: G03B27/73
Filed: 1961-04-06
Abstract: 943,105. Controlling exposure in photographic colour printing. ILFORD Ltd. March 20, 1962 [April 6, 1961], No. 12467/61. Heading G2A. [Also in Division G1] The invention relates to a method of producing a multicoloured photographic print on print material having components selectively sensitive to three spectral bands, by printing light containing light in each of the said spectral bands, in which separately, in respect of each of the spectral bands, light from a stable reference source, is allowed to fall on a photo-cell after passing through a colourselective filter chosen so that the combination of filter and photo-cell produces a response confined to the particular spectral band, and in which the sensitivity of a circuit associated with the photocell is then adjusted until the output of the circuit reaches a preselected value, the sensitivity then being held constant while light from the said reference is removed and printing light shining through a colour selective filter of the same spectral band is substituted, the exposure of the print material to light in the said spectral band being assessed by the resultant output of the said circuit. As shown light from a lamp 1 passes through a lens 2 and a colour selective filter sector 3 of a tricolour disc 4 to a mirror 5 where it is reflected through a field lens 6, a multicolour negative 7 and a lens 8 to form an image on print material 9. A partially reflecting plate 10 diverts part of the light from the lens 8 to a photomultiplier 14 through another lens 11 and a sector 12 of a disc 13 similar to disc 4. The discs 4,13 are coupled so that they may bring into operation simultaneously similar filter sectors 3<SP>1</SP> , 12<SP>1</SP> and 3<SP>11</SP>, 12<SP>11</SP>, the filter sectors transmitting red, green and blue light respectively. When a print is not being exposed, a shutter 15 obstructs light from the lens 8 and a reflector 18 moves to direct light from a reference lamp 19 through the disc 12 on to the photomultiplier cell 14. It is arranged by means not shown that the voltage applied to the dynode bleeder chain of the cell 14 is adjusted automatically so that the output current from the cell collector is held closely to a preselected value for the light in the spectral band passed by the filter 12. At the start of a print cycle the sensitivity of the cell 14 is thus already set to control exposure of the print material to light transmitted by the filter sector 3 and the negative 7. On initiation of the cycle the control of the dynode supply voltage is "paralysed", which means that the automatically determined voltage is maintained constant during the cycle irrespective of changes in the collector current, and the reflector 18 is removed so that light from the lamp 19 no longer strikes the cell 14 and the shutter 15 moves to allow the print material to be exposed. The filter sector 12 modifies the spectral response of the cell 14 to match that of the emulsion layer of the material 9 which is sensitive to light of the colour transmitted by the sector 3. By known means the collector current is used to terminate exposure by operating the shutter 15 when a preselected integral of collector current against time, proportional to the integrated light flux transmitted by the sector 3 and the negative 7, has been reached. When exposure through one filter sector is terminated the discs 4, 13 rotate to bring the other filter sectors into operation and the process is repeated until the cycle is complete. In a further embodiment (Fig. 3 not shown) the negative is illuminated by light containing all three spectral bands and three photomultiplier cells are provided each being masked by its associated fixed colour selective filter. An opaque movable flap is provided which can be moved into a position where it prevents light from the negative from reaching the cells but allows light from a reference lamp to reach the cells after passing through their filters. Operation is in a similar manner to that described above but in each part of the cycle a different one of each of the cells automatically controls the light source and thus terminates illumination of the negative by the spectral band corresponding to the filter associated with the particular cell. Specification 939,819 is referred to.